With reference to FIG. 1, a ducted fan gas turbine engine generally indicated at 10 has a principal and rotational axis X-X. The engine comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high-pressure compressor 14, combustion equipment 15, a high-pressure turbine 16, and intermediate pressure turbine 17, a low-pressure turbine 18 and a core engine exhaust nozzle 19. A nacelle 21 generally surrounds the engine 10 and defines the intake 11, a bypass duct 22 and a bypass exhaust nozzle 23.
The gas turbine engine 10 works in a conventional manner so that air entering the intake 11 is accelerated by the fan 12 to produce two air flows: a first air flow A into the intermediate pressure compressor 13 and a second air flow B which passes through the bypass duct 22 to provide propulsive thrust. The intermediate pressure compressor 13 compresses the air flow A directed into it before delivering that air to the high pressure compressor 14 where further compression takes place.
The compressed air exhausted from the high-pressure compressor 14 is directed into the combustion equipment 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines 16, 17, 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines respectively drive the high and intermediate pressure compressors 14, 13 and the fan 12 by suitable interconnecting shafts.
Labyrinth seals are used throughout a gas turbine engine, and are designed to seal two components together whilst permitting a flow of air through the sealed boundary. An example of such a seal is between a casing component of the combustion equipment 15 and a cover plate protecting components of the high pressure turbine 16. The operating temperature of the high pressure turbine components needs to be kept at a safe level to maintain component integrity. This is achieved using a labyrinth seal to permit a purging flow of cooling air from the high pressure compressor 14 to the high pressure turbine 15 components and thereby preventing ingestion of hot working gas.
Labyrinth seals have two abutting surfaces; one surface having an abradable lining and the other having a series of fins. The fins provide resistance to air flow by forcing the air to traverse around the fins along a labyrinthal path. This resistance to air flow minimises performance penalties from air leakage.
During operation, thermal and mechanical movements of the gas turbine engine structure cause relative movement of the sealed components. Thus, the distance between the two abutting surfaces of the labyrinth seal changes throughout operation. This can result in periods during operation where the lining and fins are sufficiently close that the air flow through the seal is restricted to an unacceptable level. In the case where the seal has to allow a certain level of purging air flow through the seal, restriction of the flow through the seal can lead to hot gas ingestion causing damage or failure of engine components.
A conventional solution to this problem is to position the lining and fins sufficiently apart so they never run close enough during operation to over-restrict the air flow through the seal. However, this results in periods of operation where the distance between the lining and fins is larger than necessary, and has the effect of reducing performance efficiency of the engine.